So-called "floppy" disk memory systems for "desk top" sized computers are well known in the art. Such systems employ magnetic storage disks having a diameter of either 5.25 inches or 3.50 inches. Conventional magnetic storage disks for floppy disk drives have a track density ranging from forty-eight (48) to one hundred thirty-five (35) tracks per inch (TPI). In contrast, optical storage disks for optical memory systems achieve track densities greater than 15,000 TPI. The greater track density of optical disks is achieved by the use of optical servos that maintain fine positioning of the optical read/write head over the data tracks on the disk. Typically, concentric optical servo tracks are prerecorded on the optical disk to guide the servo mechanism.
New advances in barium-ferrite magnetic media have allowed bit densities of magnetic storage disks to exceed the bit densities of optical disks. However, as mentioned above, track densities of magnetic media (48-135 TPI) are many times less than their optical counterparts. This limits the overall capacity of magnetic disks as compared to optical disks. Conventional magnetic disk systems employ a magnetic servo mechanism and magnetically prerecorded servo tracks on the disks to guide the read/write head. Magnetic servo systems cannot provide the fine positioning that optical servo systems can provide.
Recently, floppy disk systems have been developed that combine magnetic disk recording techniques with the high track capacity optical servos found in optical disk systems. Such a system is described in AN INTRODUCTION TO THE INSITE 325 FLOPTICAL(R) DISK DRIVE, Godwin, in a paper presented at the SPIE Optical Data Storage Topical Meeting (1989). Essentially, an optical servo pattern is prerecorded on a magnetic floppy disk. The optical servo pattern typically consists of a large number of equally spaced concentric tracks about the rotational axis of the disk. Data is stored in the magnetic "tracks" between the optical servo tracks using conventional magnetic recording techniques. An optical servo mechanism is provided to guide the magnetic read/write head accurately over the data between the optical servo tracks. By utilizing optical servo techniques, much higher track densities are achievable on the relatively inexpensive removable magnetic medium.
As mentioned, the optical servo pattern typically consists of a large number of equally spaced concentric tracks about the rotational axis of the disk. As disclosed in U.S. Pat. No. 4,961,123, each track may be a single continuous groove (FIG. 3), a plurality of equally spaced circular pits (FIG. 8), or a plurality of short equally spaced grooves or stitches (FIG. 9). Various methods and systems exist for inscribing the optical servo tracks on the magnetic medium.
For example, U.S. Pat. No. 5,067,039, entitled "High Track Density Magnetic Media with Pitted Optical Servo Tracks and Method for Stamping the Tracks on the Media," discloses a method for "stamping" the servo tracks on the magnetic medium. Essentially a master stamping disk is produced bearing a template of the optical servo pattern. This master disk is then pressed against the magnetic floppy disk under a pressure of several tons per square inch. The significant amount of pressure transfers the servo track pattern from the master disk to the floppy.
U.S. Pat. No. 4,633,451, entitled "Optical Servo for Magnetic Disks," discloses a method of providing optical servo information on a magnetic medium consisting of a multi-layer film. The optical servo tracks are formed on the multi-layer film by laser heating the structure to cause a reaction or interdiffusion to occur between layers. The reaction produces a reflectivity contrast of about eight percent (8%) between exposed and unexposed areas. Other methods for preparing the servo tracks are mentioned including contact printing, embossing, and lithography.
U.S. Pat. No. 4,961,123, entitled "Magnetic Information Media Storage with Optical Servo Tracks," discloses a preferable method and apparatus for etching the pattern on a disk using a focused beam of light. The magnetic disk is placed on a platen/spindle assembly and rotated. A beam of light is focused to a small spot on the spinning disk. The focussed beam has sufficient intensity to ablate the disk surface at that spot, thereby reducing the reflectivity of the surface at that spot, and as the disk rotates, a groove is produced. The beam can be left on during an entire revolution to produce a continuous groove or can be modulated on and off through one revolution to produce a stitched pattern of non-continuous grooves.
With the preferred etching method described above, the width of the etched grooves that define each servo track is a function of the energy density, ED, delivered by the incident beam to the focused spot on the disk. The energy density, ED, delivered by the beam is proportional to the intensity of the beam, I.sub.o, within the focused spot divided by the area of the spot, A.sub.o, multiplied by the linear velocity V.sub.1 of the disk at the focused spot. That is, ##EQU1## The linear velocity, V.sub.1, of the rotating disk at the focused spot is a function of the radius, r, of the particular groove or servo track being etched. That is, EQU V.sub.1 =2.pi.r.times.b,
where b is the "spin velocity" of the disk in revolutions-per-minute (rpm).
Typically, the groove width is on the order of microns and must be maintained within tight tolerances. For a single servo track, the energy density at the focused spot, and hence the width of the groove (or grooves w/ a stitched pattern), can be held constant simply by rotating the disk at a constant spin velocity, b. However, as the incident beam is moved radially of the disk to etch servo tracks at other radii, the change in radius results in a proportional change in the linear velocity of the disk surface at those radii. Therefore, if the disk rotates at a constant spin velocity, b, the energy density, ED, delivered to the surface of the disk will vary at different radii. Consequently, the groove width at different radii of the disk will vary significantly--an unacceptable condition.
One prior art method for compensating for the change in energy density at different track radii is to vary the rotational speed (rpm) of the disk to maintain a constant linear velocity despite the change in radius. Such a method is disclosed in the aforementioned U.S. Pat. No. 4,961,123 at column 10, lines 58-62. This method is disadvantageous, however, because etching must be suspended while the spindle motor settles to each new velocity. This prior art method results in lower throughput and requires high maintenance for the mechanical elements.
Another prior art method for compensating for the change in energy density is to vary the intensity of the incident beam at different radii by controlling the power supplied to the light source. Typically the light source is a laser. While varying the power supplied to the light source does alter the intensity of the beam, it also results in unacceptable beam wander and pointing stability.
Another possibility is to use a compensated neutral density wedge located in the path of the incident beam. Again, however, such a method results in unacceptable beam wander.
Thus, there is need for an apparatus and method of maintaining constant energy density at all radii of the disk, but which does not unacceptably reduce throughput and which does not require variation in the rotational speed of the disk. Additionally there is a need for an apparatus and method which does not produce undesirable beam wander. The present invention satisfies these needs.